A Case of Aortoiliac Thrombosis

Recently the Johannesburg team dealt with a case of Aortoiliac Thrombosis in a 10 year- old warmblood gelding.  The owner reported poor performance possible associated with a hindlimb lameness.  On clinical examination, the horse was found to get progressively lamer as he worked, only to return to soundness when rested for a few minutes.

Sequential nerve blocks up the limb did not resolve the lameness.  Given these findings a provisional diagnosis of aortoiliac thrombosis was made. Rectal examination revealed a grossly enlarged and firm internal iliac artery of the left hindlimb compared with the right.  Subsequent ultrasonography showed a large thrombus within the artery which was significantly impairing the passage of blood.

Image 1. Ultrasound image of the internal iliac artery showing the presence of the thrombus (red outline) within the vessel, and the reduced passage of blood (arrow).

Image 2. The true size of the thrombus removed on post-mortem. This shows the dissected out distal aorta with the thrombus (clot) protruding out to the right.

Aortoiliac thrombosis is a progressive vascular disease characterised by an exercise-induced hindlimb lameness of variable severity associated with a thrombus (clot) in the terminal aorta or as it splits into the iliac arteries.  The cause of this condition is still unclear. 

Clinical signs vary depending on the site of thrombus formation, its size, and the degree of vessel occlusion.  With mild lesions the horse may show poor performance, early fatigue or a mild hindlimb lameness.  More severe lesions may cause the horse to become distressed and start to sweat except over the hindlimbs which remain cool.  If the horse is allowed to stop it may stamp the affected limb.  Regardless of the severity of the lameness it usually disappears after a period of brief rest. 

Clinical diagnosis is made via rectal palpation of the terminal aspect of the aorta.  The thrombosed vessels may be enlarged and/or firm.  Pulses in the iliac arteries may be reduced or absent.  Ultrasonography is also useful to confirm the diagnosis and to determine the extent of the lesion. 

The cause of aortoiliac thrombosis is unknown.  Suggested causes are systemic infections such as strangles or influenza, trauma due to falling or Strongylus vulgaris (bloodworm) larvae migration.

Treatment options are very limited and usually unsuccessful.  Options for early diagnosed cases include rest and drugs which may help reduce/resolve the clot – such as aspirin.

Horses with more severe lesions are usually refractory to treatment.  Surgical removal of the thrombus is sometimes a possibility depending on its location.  However, this is associated with a high complication rate and re-occlusion of the vessels can occur. Prognosis for return to work is very poor in most cases.